Tidal power has been harnessed for many centuries. Tidal grain mills have been located on coastal inlets where seawater could be trapped by sluice gates in low dams at high tide, to be released through a mill wheel at low tide. Such mill wheels are examples of rudimentary hydraulic turbines.
Energy shortages have spurred research activity, including the construction of significant pilot projects in the area of tidal power generation using more modern turbine designs, in Normandy, France and Nova Scotia, Canada. The Normandy and Nova Scotia projects required significant infrastructure and damning of tidal estuaries. The effect of all such projects on the ocean inlet and estuaries is to interfere with tidal dynamics and disrupt marine biological ecosystems in the inter-tidal zones behind the dam.
While the pilot projects in Normandy and Nova Scotia are still in operation, they have not proven to be economical, and with the global rise in environmental consciousness, this approach has fallen into disfavor. More recent work has been focused on ‘free-stream’ machines that are less costly and far less disruptive to tidal dynamics and the affected marine ecosystems. ‘Arrays’ of such units would consist of single or multiple rows of free-stream machines deployed transversely and/or linearly in an ocean inlet or passage in the manner of a wind-farm. Indeed the term ‘turbine-farm’ has been used to describe such deployment.
In the 1920s, U.S. Pat. No. 1,493,154 to Harza disclosed a substantially horizontal axis water turbine for use in the draft tube of a low head dam, wherein the stator coils surrounded the runner or rim housing the turbine blades. Harza proposed water seals between the runner and stator. It is very difficult to maintain the integrity of water seal in a hydraulic turbine, as underwater turbines are subject to high levels of vibration, erosion and torsion due to the density and velocity of the water.
U.S. Pat. No. 3,986,787 to Mouton disclosed a uni-directional hydraulic turbine with angled blades also including a longitudinal twist. The Mouton patent teaches a deployment method of mounting the turbines under a barge in a river, with a generator on top of the barge. The Mouton patent also discloses a trash screen in front of the turbine consisting of a conical array of cables. The Mouton patent, as with most conventional hydraulic turbines, uses a hub based generator system with impact type blades which are angled with respect to the direction of the water flow. The trash screen of the Mouton patent protected only one end of the turbine and was not self-cleaning, requiring constant regular maintenance.
U.S. Pat. No. 4,163,904 to Skendrovic disclosed an understream turbine plant requiring substantial infrastructure and sealing about the single unidirectional turbine with hub generator.
Investigation into harnessing wave motion resulted in U.S. Pat. No. 4,221,538 to Wells, disclosing a uni-directional aero-foil turbine powered by the air forced through the turbine by the oscillating water column created by wave action enclosed in a floating chamber. The Wells patent disclosed a single rotor with a hub-based generator.
In the 1980's Heuss and Miller disclosed a tidal power plant in U.S. Pat. No. 4,421,990 with a fixed concrete barrage of unidirectional impact turbines with angled blades and a rim-based generator. The Heuss patent required substantial infrastructure, including a dam, draft tube and generator housing. The stator was housed in the darn or foundation at the outer rim of the runner wheel housing the turbine blades and required watertight seals.
U.S. Pat. No. 4,313,711 to Lee disclosed fixed stator blades or vanes that deflected the flow of air or water onto multiple Wells type aerofoil cross-section blades to cause efficient rotation. The Lee patent uses wave motion or wave driven air to generate power. The guide vanes are fixed and the rotors rotate at the same speed and in the same direction.
In the 1990's, Curran, and Gato ran trials on a series of different Wells type air turbines and published their results in the article: “The energy conversion performance of several types of Wells turbine designs”, Proc. Inst. Mech. Engrs. Vol 211 Part A (1997). The trials included single rotor devices with and without guide vanes, and dual rotor devices with the rotors rotating in the same direction and counter-rotating. Although Curran and Gato did not investigate the effect of dual counter rotating rotors with guide vanes, they concluded that two rotors are more efficient than one, that counter-rotating rotors provided a higher damping ratio and improved post-stall performance than uni-directional pairs, and that inlet and outlet guide vanes provided reduced tangential kinetic energy losses compared to those units without vanes.
The following papers are also of interest regarding a vertical axis turbine and a feasibility study on sub-sea power generation from tidal currents:                Davis, Barry V. (1997) Nova Energy Ltd. “A Major Source of Energy From the Worlds Oceans” IECEC-97 Conference, Jul. 31, 1997, Honolulu.        Davis, Barry V. (1980). Nova Energy Ltd. “Hydraulic Turbine Trials” Report No. NEL 002. DSS Contract No. OSX-00043.        
Vauthier, in U.S. Pat. Nos. 6,168,373B1, 6,406,251, and 2002/0088222A1 disclosed a floating, uni-directional and bi-directional lightweight dual ducted turbine hub-generator unit suitable for either tidal or river deployment. The dual, side by side rotationally uni-directional, turbine swings freely with the water current. The turbine therefore must swing to the direction of the current as it accepts only uni-directinal water flow. The bi-directional unit is moored at both ends and thereby kept in the line of current regardless of the direction of water flow. Additional features include stabilizer fins on the housing, and an augmentor ring at the downstream end of the housing to deflect external water flow thereby creating a venturi effect at the outfall of the housing and presumably accelerating the flow of water through the turbine. The turbine blades are of the angled, conventional type and guide vanes are not used.
In the fall of 2001 Vortec Energy Limited of New Zealand published an “Information Memorandum” disclosing a wind turbine using a diffuser ring to create a low-pressure region downstream of the turbine rotor. The preferred embodiment of the Vortec unit is a huge 50 m diameter and greater wind turbine deployed either on or offshore. Barge, pole and block mounted sub-sea units were contemplated, but not developed. The Vortec Memorandum also suggests the possibility of rim generation to eliminate the need for large center body structures and hub mechanisms. In PCT Publication WO 01/06122 A1 to Fox et. al, owned by Vortec, advantages of a slotted, aerofoil cross section blade in a turbine are disclosed.
The current technology for hydro turbines is unsatisfactory due to the large number of moving parts and complexity of manufacturing, installing and maintaining turbines in the corrosive salt water environment.
There is a need therefore for an efficient hydraulic turbine generator unit, which can harness tidal energy with a minimal environmental impact. A turbine with a minimum number of moving parts which optimizes energy conservation by minimizing friction and flow losses, and can be manufactured, installed and maintained without substantial infrastructure costs is required, and a simple generator free of moving parts such that maintenance requirements are minimized. The present invention provides a ducted, flooded rim generator, bi-directional turbine having two or more coaxial counter rotating rotors with augmenter skirt that overcomes the disadvantages of the prior art.